The present invention relates to a hydraulic control system particularly adapted for tractor-implement correlation. More particularly, the present invention relates to a control valve that is actuated in response to the draft load imposed on the tractor by a ground engaging implement to control the position of the implement in accordance with such a load.
Hydraulic systems of this type have taken various forms. One type of system is disclosed in North, U.S. Pat. No. 3,235,010, portions of which are consistent with this disclosure being incorporated herein by reference. The North patent, which is assigned to the assignee of the present invention, discloses a control valve wherein accurate implement positioning is obtained by providing relatively small fluid flow interiorly of the hydraulic system when small incremental changes are necessary. The valve is designed so as to provide increased flow when a greater change in implement positioning is necessary.
The control valve disclosed in the North patent is what is commonly referred to as an open-center system. An open-center system incorporates a fixed displacement pump that is capable of delivering a maximum given flow required for the entire system and the control valve associated therewith connects the pump directly to the reservoir when the valve is in the neutral position. As expected in a system of this type, the pump is constantly delivering full flow at low pressure when the control valve is in a neutral position which results in energy loss. Furthermore, in most hydraulic control systems, the system is designed so as to provide a maximum flow rate which is seldom needed for operation of the unit being controlled. Thus, in most instances at least a portion of the power is wasted since the system seldom demands the maximum flow of the pump and the excess flow is bypassed, which means that further energy is lost.
While an open-center system has various drawbacks, to date, most manufacturers of agricultural implements still utilize such a system because no suitable acceptable alternate is available.
Another system that has been used in what is known as a closed-center system. The closed-center system or circuit incorporates a variable displacement, pressure compensated pump capable of a given maximum flow rate and the system is always operated at a maximum pressure. In this system, power is again wasted when less than the full presence is required for oerating the controlled unit. In this system, the pump is constantly operating at full pressure which may result in heating of the fluid when the system requires a lower pressure than maximum for extended periods of time.
Quite recently a third type of system has been developed which may be termed as a "load-sensing or pressure-flow compensated (PFC)" hydraulic system. This type of system is generally disclosed in U.S. Pat. No. 3,486,334. The hydraulic circuit disclosed in this patent consists of a variable displacement pump, the output of which is controlled by a pressure compensating member so that the pump produces the required flow at a pressure of approximately 200-400 PSI above required pressure for delivery to a controlled unit. This type of hydraulic circuit delivers low flow at low pressure for minimum pump wear, flow losses and energy losses when the valve is in a neutral position. This type of a system has the unique advantage of being capable of maintaining high pressures without delivering a large volume of fluid and also delivering larger volumes of fluid at lower pressures.
Plate et al. U.S. Pat. No. 3,604,312 discloses a hydraulic control circuit for a tractor-implement hydraulic system which incorporates the "load-sensing" hydraulic system. However, such a system has not found any large measure of commercial success. It is believed that part of the reason for the lack of commercial success in the draft control art is the complicated arrangement of the control valve that is required for incorporation into the draft control system. Some of the shortcomings of the control system of the type disclosed in the Plate et al. patent are that the system is slow to respond to draft load changes. Also, because of the particular valve construction, it is possible for some leakage to occur and result in reducing the pressure in the hydraulic cylinders that are associated with the implement when such pressure reducing is not desired. Furthermore, such leakage will result in the implement lowering when the system is in a neutral position and the vehicle is shut down.